The present disclosure relates to a semiconductor laser driving circuit and a semiconductor laser device including the same.
In recent years, the speed-up of the optical communication has been rapidly progressed. Thus, a giga b/s-optical communication system is previously put to practical use. In addition, a semiconductor laser diode such as a Vertical Cavity Surface Emitting Laser (VCSE) is used as a light source for a short-wavelength optical transceiver of the high-speed optical communication system.
An operation of the semiconductor laser diode is controlled by a semiconductor laser driving circuit. The semiconductor laser driving circuit controls the operation of the semiconductor laser diode in accordance with a magnitude of a drive current supplied to the semiconductor laser diode, thereby expressing binary digital data. Normally, the semiconductor laser driving circuit supplies a large drive current I1 to the semiconductor laser diode when data “1” is expressed, while the semiconductor laser driving circuit supplies a small drive current I0 to the semiconductor laser diode when data “0” is expressed.
When the semiconductor laser driving circuit supplies the large drive current I1 to the semiconductor laser diode, a voltage V1 is produced across opposite terminals of the semiconductor laser diode. Although the voltage V1 is changed depending on the environment such as a temperature, the voltage V1 rises up to about 2.4 V in some cases. Therefore, a configuration with which a voltage of about 2.4 V can be applied to a semiconductor laser diode is required for the semiconductor laser driving circuit. As a result, this impedes either the lowering of the voltage or the speed-up of the semiconductor laser driving circuit.
Then, a laser diode driving circuit described in Japanese Patent Laid-Open No. Hei 11-340561 sorts drive currents I1 and I0 into a bias current, and a modulation current generated in a switching portion, and supplies both of the bias current and the modulation current to the semiconductor laser diode. As a result, a transistor in a switching portion which generates the modulation current is prevented from being saturated, and thus the laser diode driving circuit can carry out the high-speed operation even at the low voltage.